1. Field of the Invention
The present invention relates to sintering nuclear fuel and more particularly refers to a new method and apparatus for sintering pellets of nuclear fuel oxides and mixtures of nuclear fuel oxides in a reducing furnace atmosphere.
2. Description of the Prior Art
Nuclear fuel is understood here to mean uranium, plutonium and thorium, alone or in mixture. For the sake of simplification, however, only uranium dioxide will be mentioned in the following discussion.
Nuclear fuel pellets are manufactured in a known manner by pressing powdered UO.sub.2+x in which oxygen is in stoichiometric excess of the dioxide and/or mixtures of UO.sub.2+x containing oxygen in stoichiometric excess and powdered PuO.sub.2 to form pressed blanks of various geometry. These pressed blanks or pellets are produced either without the addition of binder and lubricating agents in pressing tools automatically lubricated with lubricating oil of differing origin, or with the addition of binder and lubricating agents such as, for example, Zn stearate, Zn behenate, paraffins or similar materials.
After pressing, the formed blanks or pellets are placed in highly heat-resistant transport containers, called transport boats. The laden boats are pushed through a resistance-heated push-through sintering furnace lined with highly refractory blocks, where the stoichiometric excess oxygen of the UO.sub.2+x is first reduced to stoichiometric UO.sub.2.00 in reducing gases such a hydrogen and/or rare gas/hydrogen or nitrogen/hydrogen mixtures; and the pressed blanks or pellets sintered at temperatures of about 1700.degree. C. to form dense, stable pellets.
In the special case of manufacturing sintered UO.sub.2 /PuO.sub.2 bodies for light-water reactors and breeder reactors, a gas mixture which maximally contains only 8% hydrogen is used for the reduction of stoichiometric excess oxygen UO.sub.2+x for safety reasons (possible formation of explosive oxygen-H.sub.2 mixtures).
As a result of this lower hydrogen concentration in the gas mixture, the reduction potential of the gas mixture (as expressed as partial free enthalpy of the oxygen and thus, in the system H.sub.2 /H.sub.2 O, proportional to the H.sub.2 /H.sub.2 O ratio) is greatly lowered as compared to pure hydrogen. This lower reduction potential leads to a considerable lengthening of the reduction time and, with the sintering furnace following directly, to an equivalent lengthening of the sintering time.
The reaction water produced in the reaction lowers the reduction potential further, as the water concentration in the gas increases. For the gas mixture still to have a reducing effect, the H.sub.2 /H.sub.2 O partial pressure ratio should not become lower than 10:1. In order to compensate for this change of the reduction potential, which itself is again proportional to the amount of oxide reduced per unit time, dry fresh gas can be introduced into the furnace. For throughputs of, say, 12 kg UO.sub.2.2 /hour, a total of 35 m.sup.3 of gas mixture is flushed through the furnace per hour, so that the ratio H.sub.2 /H.sub.2 O does not drop below 10:1.
In sintering the UO.sub.2 /PuO.sub.2 fuel pellets, an overall stoichiometric oxygen deficient oxide, caused by the reduction of the Pu(IV) to Pu(III), can be produced at the prevailing high temperatures. Depending on the intended application, whether in a light-water reactor or in a breeder reactor, either a stoichimetric or a stoichiometric deficient oxide is desired. To adjust the respective desired stoichiometry, it is necessary to adjust a respectively different reduction potential in the high-temperature portion of the furnace. This is adjusted by humidifying the fresh gas entering the furnace to previously calculated water concentrations.
The requirements which are thus obtained for the process technique with respect to the reduction potential in the reduction and sintering portion of the push-through furnace are therefore contradictory.
If the inexpensive nitrogen/hydrogen mixture is used for the reduction and the sintering, one finds excessive contamination of the nuclear fuel by nitrogen. This can be reduced by a heat treatment at T&gt;1000.degree. C. in rare gases or rare gas/hydrogen mixtures. Of necessity this leads to using only the expensive rare gas/hydrogen mixture as the reduction and sintering gas, if only one gas mixture is used for both parts of the furnace.